"Google argues that SafeSearch is designed to err on the side of caution."

Report criticizes Google's porn filters | CNET News.com

Er... so what? Imagine if it didn't err on the side of caution. Any system such as this is either optimized for relevance or retrieval, and Google's is optimized for relevance.

The best solution to filtering is an editor/software hybrid. Editors cost money, so the only practical way of dealing with this is to have site owners become incentivized editors and submit to a growing list of sites which wish to be removed from a software determined blacklist.